The Flow Cytometry Core will meet the special needs of the Program Project members. The availability of quality sorting and analysis capabilities is essential for the success of the research efforts proposed in this Program Project Grant. We will be active users of both the MoFlo cell sorting and LSR II fluorescence analyzer in the studies dealing with: sorting of small numbers of cells based on either cell surface or transduced gene markers;and multiparameter analyses to detect biochemical and biological changes in rare populations of lymphocytes as well as whole populations transduced with various genes. In many cases, the individual program directors will need access to machines that will require triple or even quadruple staining for the sophisticated experiments outlined in the proposal. The specific aims of this core are 1. To provide high speed cell sorting to isolate cells that can be used for functional and biochemical studies;2. To provide analytical flow cytometry using a state-of-the-art LSR II to perform multiparameter analyses and 3. To provide innovative approaches to study immune tolerance using flow microscopy. The availability of the core ensures that the PPG investigators will have access to this essential facility on the UCSF campus.